NiCo₂O₄/RGO Hybrid Nanostructures on Surface-Modified Ni Core for Flexible Wire-Shaped Supercapacitor

In this work, we report surface-modified nickel (Ni) wire/NiCo₂O₄/reduced graphene oxide (Ni/NCO/RGO) electrodes fabricated by a combination of facile solvothermal and hydrothermal deposition methods for wire-shaped supercapacitor application. The effect of Ni wire etching on the microstructural, surface morphological and electrochemical properties of Ni/NCO/RGO electrodes was investigated in detail. On account of the improved hybrid nanostructure and the synergistic effect between spinel-NiCo₂O₄ hollow microspheres and RGO nanoflakes, the electrode obtained from Ni wire etched for 10 min, i.e., Ni₁₀/NCO/RGO exhibits the lowest initial equivalent resistance (1.68 Ω), and displays a good rate capability with a volumetric capacitance (2.64 F/cm³) and areal capacitance (25.3 mF/cm²). Additionally, the volumetric specific capacitance calculated by considering only active material volume was found to be as high as 253 F/cm³. It is revealed that the diffusion-controlled process related to faradaic volume processes (battery type) contributed significantly to the surface-controlled process of the Ni₁₀/NCO/RGO electrode compared to other electrodes that led to the optimum electrochemical performance. Furthermore, the wire-shaped supercapacitor (WSC) was fabricated by assembling two optimum electrodes in-twisted structure with gel electrolyte and the device exhibited 10 μWh/cm³ (54 mWh/kg) energy density and 4.95 mW/cm³ (27 W/kg) power density at 200 μA. Finally, the repeatability, flexibility, and scalability of WSCs were successfully demonstrated at various device lengths and bending angles.